Allow for automatic workload installation by specifying required workloads in `global.json`

### Problem Description

Currently, it is not possible to automate the installation of a required workload for projects. It would be good to not have to remind contributors to a project to run `dotnet workload install ` before they can build.

The feature would additionally be useful for CI jobs, which would no longer have to manually install required workloads.

### Proposed Solution

An entry in the `global.json` file specifying the required workloads (and, potentially, their version) which the SDK then picks up on and installs before a restore/build would be ideal:
```json
{
"sdk": {
"version": "8.0.404"
},
"workloads": ["wasi-experimental"]
}
```
or:
```json
{
"sdk": {
"version": "8.0.404"
},
"workloads": {
"wasi-experimental": {
"version": "8.0.404"
}
}
}
```